The Investigation Of Some Phenomena About Quantum Coherence In A Three-level System

The thesis focused on the systematically theoretical study on some phenomena about quantum coherence: Lasing without inversion (LWI), subluminal and superluminal et al.. Firstly, we proposed a new three-level model used to realize LWI, in which we analyzed some basic parameters which can affect the realization of LWI. Secondly, we investigated the change of the group velocity from subluminal to superluminal (optic knob). This paper is organized as six sections according to the contents and results: section 1 is a survey; section 2 is a review on the semiclassical theory of light-matter interaction; in section 3, Electromagnetically induced transparency (EIT) which is an interesting phenomenon about quantum coherence is investigated. At the same time, LWI, subluminal and superluminal, as two important application of EIT, are simply introduced; The following three sections are the introductions of the author's work.In the first Chapter, We give an introduction of the research background and the research purpose of my thesis.In Chapter 2, Taken the two-level atomic system for example, the semiclassical theory of laser-matter interaction, which is the basis of the following works.In Chapter 3, Based on the semiclassical theory of light-matter interaction, we get EIT, an interesting phenomenon about quantum coherence. Here, we emphasize its production mechanism and application. During the application, we focus on LWI, subluminal and superluminal, which we are interested in.In the following Chapter, we mainly analyze the phenomenon of LWI in a three-level system. Here, we propose a new three-level model used to realize LWI. The incoherent pumping or the decay between two upper levels must be considered in others' models. While, in our model, we can realize LWI only when spontaneously generated coherence (SGC) between two upper levels is considered. During this process, SGC term is not always impulse the occurrence of LWI, it can cumber the realization under some condition.At the same time, we also find that the generation of the probe field is not only due to itself, but also to another driving field, this can be viewed as a new coherence.In Chapter 5, we again investigate the effects of the incoherent pumping in a three-level system used to realize LWI. We find that, by the numerical results, both the incoherent pumping and SGC are necessary in the process of making the system become sensitive to the relative phase. Here, there exists an incoherent pumping corresponding to the extremum of the amplitude of the probe gain.In the last chapter, the change of the group velocity is discussed in a V system. We find that the change of incoherent pumping can affect the slope of the dispersive curves. Here, there exists an incoherent pumping rate which makes the group velocity reach the extremum. The existence of SGC is just the cause for the occurrence of the extremum. During the whole process, SGC can be regarded as a optic knob which is used to manipulate light propagation from subluminal to superluminal.